What Happens When You Dip Black Steel in Hydrochloric Acid?

Black steel, that steel I mentioned in an earlier post, reacts when exposed to hydrochloric acid (HCl). In fact, most steel does. The other day I had the chance to put a small carbon steel pipe in a concentrated HCl solution (about 3 N, or 3 molar). The picture to the left shows what is happening.

Tons of bubbles formed! This was expected since iron will react with HCl. Some people use HCl to prepare black steel for welding. The reaction will definitely strip away the “black” steel covering. That black layer is just magnetite and some other impurities. However, the reaction may compromise the integrity of the steel, especially if there are impurities. Just keep that in mind!

Anyway, the reaction proceeds by the following:

Fe(solid) + 2H+ –> Fe2+ + H2(gas)

As you can see, hydrogen gas is being produced. You know it’s a fun experiment when a flammable gas is being created. In fact, you could light the gas on fire – just be very careful not to ignite the gas in a closed container and, as always, wear proper safety gear.

What’s essentially happening in this particular reaction is expedited corrosion. Many chemical plants use black/carbon steel and, as you may suspect, many chemical plants are piping around hydrochloric acid. There are many studies available which discuss the corrosion of steel as a result of coming into contact with HCl.

See the image to the left for a view of the steel before and after the simulated corrosion. Note that the more grey looking pipe segment is the corroded steel.

The corrosion of steel by HCl turns out to be a first order reaction, where the rate of corrosion (or in this case, the rate of visible hydrogen production) is directly proportional to the molar concentration of HCl.